1. Field of the Invention
The present invention relates to an image printing apparatus and, more particularly, to an image printing apparatus such as a printer, facsimile, or copying machine which increases the print width using a plurality of printheads.
2. Description of the Related Art
Image processing apparatuses such as a printer and copying machine adopt image printing apparatuses of thermal printing type, heat transfer printing type, inkjet printing type, and the like. These printing methods use a printhead in which many printing elements for forming print dots are linearly arranged at equal intervals. Methods of forming an image with such printhead include two-valued printing in which each print dot takes only two, printed and unprinted states and halftone is expressed by the density of print dots, and multi-valued printing in which each print dot can take a plurality of (generally three or more) density levels and more smooth halftone expression can be realized.
Upon manufacturing a printhead, as it becomes longer, the manufacturing yield of non-defectives becomes lower in the manufacture to raise the cost. To prevent this problem, there is proposed a method of increasing the print width using many printheads whose print width is short at low manufacturing cost.
For example, Jpn. Pat. Appln. KOKAI Publication No. 6-255175 discloses a method which makes a joint inconspicuous. According to this method, a plurality of printheads which print image data are arranged (linked) on a substantially straight line so that their ends overlap each other. In supplying image signals to the printheads, the switching positions of the printheads at joints are irregularly changed for each scanning line.
In the prior art, the density of a printed image may become higher or lower than a predetermined one depending on the installation position of the printhead at the joint between printheads. This degrades the image quality.
In multi-valued printing capable of expressing a plurality of density levels by one print dot, a higher-quality image is formed in comparison with two-valued printing. A joint stands out when an image is formed only by dividing the print range of each printhead into a random shape at the joint between two printheads.